Scientists have developed a more affordable and efficient method of printing copper micropatterns on glass surfaces.
The micropatterns can now be printed 100 times faster for use in various devices such as chemical sensors, flexible electronics, and anti-theft systems.
Motherboards and printed circuit boards are covered with several conductive micropatterns that are connected to each other and the board’s other elements. The patterns themselves can be created via methods such as lithography – a deep printing technique wherein the pattern is transferred from one coil to another and then electrically deposited.
However, lithography is time-consuming and requires thorough preparation: it takes around 40 hours to create a pattern. Other methods require expensive equipment and large quantities of solvents in order to cover the material with a copper layer, none of which is economically viable in small-batch production.
More efficient methods involving direct laser writing and subsequent chemical deposition have been developed over the years. These methods are faster and more environmentally-friendly, as well as flexible in terms of the micropattern that can be produced. However, they still require an expensive femtosecond laser and the use of toxic solvents, and can also be destabilised by external conditions.
Researchers from ITMO University and the Institute of Chemistry of St. Petersburg State University have therefore developed a new laser-induced copper deposition method from deep eutectic solvents that combines the advantages of existing methods while at the same time making the printing process cheaper and 100 times faster. The process, described in Nanomaterials, uses a commercially available nanosecond fibre laser with a maximum average power of 20W.
A multi-stage process
The writing of micropatterns is done in several stages. First, the researchers clean a glass surface from dirt, dust, and fingerprints, because any residue left on the surface can affect the following chemical processes, as well as the interaction of the laser and the surface. Then, a laser-induced microplasma technique is used to prepare the surface. This means that the glass is put on a titanium plate and then processed with plasma generated by laser radiation. As a result, a dark patina is created from titanium’s combustion products and oxides in the form of small particles that produce additional crystallisation centres. This process changes the glass surface, increasing its adhesion.
After that, a deep eutectic solvent made from tartaric acid, copper acetate, and choline chloride is put on the glass. This composition of the solvent is cheaper and more environmentally-friendly than that used in existing methods; moreover, it is able to quickly decompose metal salts in high concentrations in a higher temperature. This is necessary for more efficient synthesis of electric wires from copper on glass. At the same time, the deep eutectic solvent can be stored longer and used more sparingly compared to its organic and water-based counterparts.
The glass is put on a titanium plate and then treated with laser-generated plasma. Auxillary particles then form on the surface, increasing its adhesion. (Image: Dmitry Grigoryev)
In the next step, another layer of glass is put over the solvent-lubricated surface in order to get rid of air bubbles, allow the solvent to spread more evenly, and keep it inside the processing area. When subjected to high temperatures and laser radiation, the solvent thickens, thus triggering the so-called Noah effect, which is when the liquid spreads from the centre of the glass to its edges. The researchers have nicknamed this design a ‘sandwich’.
Finally, at the last stage the sandwich is once again subjected to laser radiation, forming on the lower glass the micropatterns previously designed using graphic software. They can take any shape depending on the researchers’ current plan: for instance, circles and lines for SPEs (screen printed electrodes) or complex shapes with many angles for multifunctional sensors.
A wide range of applications
The new method can be used in different fields – for one, in the development of chemical sensors. These devices react to changes in the contents and volume of components in a chemical environment.
The method can also be used to create RFID markings, a common theft-prevention technology. As the new method enables the creation of very complex outlines, this will make it possible to conclusively tell which brand store the item was taken from.
However, according to the researchers, these are not the only applications: ‘Any electronic circuit board with a worn-off pattern is usually disposed of; however, our method will allow us to refurbish it by recording it once more,’ said Ekaterina Avilova a researcher at ITMO’s International Laser Micro- and Nanotechnologies Laboratory and first author of the Nanomaterials paper. ‘We are also hoping that our method will prove useful for flexible electronics, because our technique can be used to design flexible screens or physiological sensors that are stuck on the body to measure the body’s water balance or level of sugar. This design can be helpful in treating diabetes.’
In the near future, the team is planning to improve the method with the help of several scientific groups. In this interdisciplinary study, chemists will be responsible for adjusting components’ concentration or replacing components to see how it affects the method's efficiency. At the same time, physicists will study the laser radiation properties, such as power, frequency, and wavelength. It is also important to take into account the various aspects of pattern-writing: from the distance between its lines to the writing sequence, number of treatments, and so on. All of this will help the scientists increase the speed of micropattern formation and increase its quality.